Geoacoustic Parameters Research Articles

Geoacoustic models for the Icelandic Basin

Geoacoustic models inferred from amplitude versus range data at 220 Hz are presented for three locations in the Icelandic Basin. The data were obtained using a deep‐towed pulsed cw source and two receivers anchored near the bottom. For ranges out to 4800 m, the data were analyzed using an iteration of forward models technique in which the parabolic equation method and a Hankel transform method were used sequentially to compute acoustic fields for different bottom parameters until best fits to the data were obtained. The inferred geoacoustic models consist of a sediment layer containing a positive, linear sound‐speed gradient overlying an isovelocity sub‐bottom. The geoacoustic parameters include the layer thickness, the sound‐speed gradient, and the sound‐speed discontinuities at the water–bottom and sub‐bottom interfaces. The density and attenuation of the structures are also determined. The geoacoustic models are substantiated by other types of measurements, which include piston coring, 3.5‐kHz seismic profiling, and in situ sediment profiling. These additional measurements, combined with the 220‐Hz results, yield a consistent picture of the geoacoustic nature of the three areas. Specifically, the highly coherent character of the 220‐Hz data, the clearly visible sub‐bottom layering in the 3.5‐kHz records, and the low values of attenuation are related to the fine‐grained nature of the sediment at two of the sites. At the third site, the presence of coarse sediment provides an explanation for the major incoherent component in the 220‐Hz data, the poorly discernible layering in the 3.5‐kHz records, and an anomalously high value of attenuation.

Geoacoustic models for propagation modeling in shallow water

Acoustic propagation in shallow water is viewed as a guided‐wave phenomenon, with the sea surface and seabed forming the boundaries. At subkilohertz frequencies, the acoustic properties of the seabed to a depth of several wavelengths can have a strong effect on propagation. The computer modeling of propagation requires estimates of such parameters as sound speed, density, attenuation, and layer thicknesses, which collectively are called the geoacoustic model of the seabed. Direct measurement of these quantities is difficult, and methods must be devised to infer these values from other experiments, often employing acoustic techniques. At DREA, we have adopted the approach of independently determining as many geoacoustic parameters as possible, and adjusting less precisely known parameters within reasonable limits to effect an agreement between theory and experiment. To this end, we have used sub‐bottom reflection profiles to determine sediment types and layer thicknesses, large and small scale seismic refraction experiments to estimate sound speeds, and processing of sub‐bottom vertical reflection data to estimate volume attenuation. Techniques used by other researchers will be reviewed. Examples of geoacoustic models and comparisons with experiment will be presented for shallow water sites on the Scotian Shelf and the southwestern approaches to the English Channel.

Sound channels in surficial marine sediments: Hamilton, E.L. Vol 48, No 5 (November 1970) pp1296–1298

The monitoring, assessment and prediction of dynamic processes in shallow water constitute an attractive challenge. The availability of targeted observations enable high-resolution ocean forecasting to develop the 4D environmental picture. In particular, range-resolving acoustic tomography data constitute an effective way to reduce the non-uniform distribution and sparsity of standard hydrographic observations. In this paper a Kalman filtering scheme is investigated for tracking the time variations of a range-dependent sound-speed field in a vertical slice of a shallow water environment from full-field acoustic data and a propagation model taking into account the acoustic properties of the seafloor and subseafloor. The basic measurement setup for each radial of a tomography system consists of a broadband, multifrequency sound source and a vertical receiver array spanning most of the water column. The state variables represent the main features of the sound-speed field in a low dimensional parameterization scheme using empirical orthogonal functions. To test the algorithm acoustic data are synthesized from ocean model predictions obtained in support of the MREA/BP07 experiment southeast of the island of Elba, Italy. Bottom geoacoustic parameters obtained from previous acoustic inversion experiments are input to a normal mode propagation model as a background dataset. Additional data such as sea-surface temperature data from satellite or in situ hydrographic observations provide a priori approximate information about the range dependency of the subsurface structure and an estimation of the sea-surface sound speed. The evolution of the entire sound-speed field in the vertical slice is then sequentially estimated by the inversion processor. The results show that the daily space and time variations of the simulated sound-speed field can be effectively tracked with an extended Kalman filter. The depth-integrated sound-speed error (RMS) remains lower than 0.3 m/s (0.09 °C) when the benchmark environment is completely determined in the parameter space and lower than 0.7 m/s (0.22 °C) for an approximate environment parameterization.